Hydraulic fluids



Patented Feb. 20, 1951 HYDRAULIC FLUIDS Kenneth E.. Walker, Wilmington, Del.,. assignoi'.

to E. I. du Pont de Nemours & Companyt Wilmington, Del., a corporation. of Delaware No. Drawing. Application December 29, 1947, Serial No. 794,455;

12 Claims.

This invention relates to compositions of matter, and more particularly fluids having use in hydraulically operated apparatus such, for example, as hydraulic brakes, hydraulic clutches, hydraulically operated airplane service mechanisms and the like.

Various proposals have. been made to use mixtures of polyoxyalkalenecompositions with castor oil, glycerine, and similar lubricants for hydraulic transmission of power such as is used for activating the pressure-operated elements of brake systems, absorber systems, activating mechanisms, automotive clutches and similarly operated devices. Many of the fluids previously proposed for these purposes, however, have a number of disadvantages, for example, as cor rosive action on the metals used, excessive swelling and deteriorating action on the rubber parts, tendency towards solidification under low temperature conditions, and poor lubricating properties for the moving parts of the system. All of these and similar disadvantages limit the utility and commercial utilization of such fluids.

An object of the present invention is to provide hydraulic fluids containing none of the? aforesaid disadvantages and, moreover, to provide an improved hydraulic fluid characterized by having a wide 'range of utility under the extreme operating temperatures encountered in service mechanisms used in submarines, automobiles, airplanes, etc.

Another object of the invention is to provide a non-petroleum hydraulic fluid. containing a mixture of organic compounds, one of which provides suitable lubrication and viscosity charobjects and advantages of the invention will.

hereinafter appear.

Improved hydraulic fluids are produced in accord with this invention by mixing a lubricant comprising essentially a diester of. a polybasic yijacid with a solvent or solvent mixture. miscible I -therewith. The fluids resulting from such mixtures are characterized particularly by a relatively narrow range of viscosities at high and low temperature, absence of any corrosion or d'ecompositionof ferrous or non -ferrous. metal parts, substantially no swelling of artiflcal or synthetic rubber, and no gasification or so1idifica-- tion. under the extreme temperature conditions encountered in. automobile or airplane opera-- tion.

The lubricants employed in accordance withthis invention include more particularly; the aliphatic or aromatic. esters of the polybasic acids, and more especially theesters of acids having at least 3 carbon atoms, for example,v the esters of malonic, succinic, glutaric, diglycolic, adipic, pimelic, suberic, and the like aliphatic saturated dibasic acids. The esters of the aromatic dibasic acids can, likewise, be. used to advantage such, for example, as esters of phthalicand terephthalic acids.

Any alkyl ester of the aforesaid acids may be: employed where rubber swelling is not of first. importance as, for example, methyl, ethyl, m and isopropyl, n and isobutyl, and the higher, straight and branched chain monoor dialkyl; esters, the diesters may be symmetrical or unsymmetrical such as the methyl ethyl adipate, ethyl butyl glutarate and similar unsymmetrical estersi The alkoxy alkyl esters of the dibasicacids may,;likewise, be employed, for example, methoxymethoxy ethyl, ethox'ymethoxy ethyl and the higher alkoxymethoxy ethyl, ethoxymethoxy ethyl and the higher alkoxymetliox y alkyl esters, all of which contain a formal group. The aryl esters of the dibasic acids are, likewise, suitable such, for example, as benzoyl, toluyl, phenyl, and like esters.

The glycol ether esters may, likewise, be used and examples of such esters may be included, for example, the dibasic acid esters of monomethyl ether of ethylene glycol, mono'ethyl ether of ethylene glycol, and the monomethyl and monoethyl ethers of the propylene glycols. These esters will not have free hydroxyls but free ether groups, and while there is a tendency in such cases for somewhat more swelling oi rubber, nevertheless, these esters are especially recommended when rubber swelling is not or primary importance.

I Ithas been found, however, that the: most outstanding esters with respect to lubrication characteristics which are superior over a wide temperature range, are provided by the glycol esters: sum, for? example, as ethylene. glycol;- d ethylene glycol: LZ and LIB-propylene glycol;

rubber swelling, and this characteristic is inher- I out in the esters having free hydroxyl groups as, for. example, bis(ethylen .glycol) glutarate, bis(propylene glycol) glutarate, bis(ethyle'ne glycol) adipate, and equivalent esters of other dibasic acids.

Along with the aforesaid esters of a dibasicv acid, there is used a solvent for the ester, and more particularly for this purpose, are employed aliphatic alcohols having relatively low molecular weight such, for example, as diacetone, propyl, isopropyl, butyl, isobutyl, amyl, octyl, nonyl, decyl, and dodecyl alcohols, the formal alcohols such as methoxymethoxyethanol, ethoxymethoxyethanol, di-beta-hydroxyethyl formal and similar alcohols and, the ether alcohols such as methyl ether of ethylene glycol and methyl ether of diethylene glycol. As solvent with the alcohols may be used such esters that contain hydroxyl groups such as, methyl tartrate, ethyl lactate, methyl glycolate, and the like. Solvents having a boiling point of at least 200 F. are especially preferred.

The proportion of lubricant to solvent may vary over a comparatively wide range such, for ex ample, as from to 70% by volume of the lubricant based on the solvent used with a preferred ratio depending upon the dibasic acid ester employed between 40 and 60% of the ester based on the volume of solvent.

dimethylglutarate with 0.5% zinc borate catalyst) per cent by volume 40 and Beta-(methoxymethoxy) ethanoldo 60 miscible when mixed together, were also miscible after 6 days at 40 F.

Example 3 Bis(propylene glycol) adipate (prepared in 95% conversion from 1,2-propylene glycol and dimethyl adipate with 0.5% zinc borate catalyst) per cent by volume V and Isobutanol do miscible when mixed together, were also miscible after 5 days at F. with the isobutanol.

Example 4 Bis(dipropylene glycol) glutarate (prepared in 66% conversion from dipropylene glycol and dimethyl glutarate with 0.5% zinc borate catalyst) per cent by volume 40 and r Beta(methoxymethoxy) ethanol do 60 miscible when mixed together, were also miscible after 6 days at 40 F.

Example 5 lyst) per cent by volume 40 and Isobutanol do 60 fluid, and miscible after 5 days at -40 F.

Example 6 Bis(propylene glycol) glutarate (prepared The following examples illustrate preferred hydraulic fluid compositions:

Example 1 V in 95% conversion from 1,2-propylene glycol and dimethyl glutarate with 0.5% potassium hydroxide catalyst) per cent by volume 40 and Isobutanol do 60 had the following properties compared to the test catalyst) per cent by volume 40 limits of the Society of Automotive Engineersand specification for moderate duty. hydraulic brake Isobutanol do fluid:

SAE test limits ggg gg g Viscosity (Kinematic) at 20 F 1000 centistokes maximum 330 centistokes.

3.5 ce tis'tokes minimu 230 min.i. Not less than 0.005 in. Not more than 0.050 in. diameter gain Metal parts shall be free from excessive wear, corromm, or galling.

l 4.7 centistokes.

condition with no corrosion or galhng,

were refluxed hour in order to produce a fluid .miscible after 6 days at 40 F.

E xample 7 Bis(propylene glycol) glutarate (prepared in Viscosity at 100 F centistokes i 6.03 conversion from 1,2-propylene glycol Viscosity at 40 F do 511 and dimethyl glutarate with 0.5% potas- Emm le 2 sium hydroxide catalyst) p per cent by volume 40 Bis(ethylene glycol) glutarate (prepared in. and

' 95% conversion from ethylene glycolan'd Beta(methoxymethoxy)ethanol do- 60 Metal parts in good had the following properties as compared to the test limits of the Society of Automotive Engineers specification for heavy duty hydraulic brake fluid:

Fluid from SAE tcst limits Fxample 7 Viscosity (kinematic) at F Viscosity (kinematic) at 130 F..." Boiling point, deg. F Rubber swelling After 120 hr. immersion in fluid at 158 F. Freezing point (6 hr. at 60 F.)

2200 centistokes maximum.

3.5 centistokes minimum..-

Not less than 0.005 in Not more than 0.050 in. diameter gain Shall begin to flow 5 see. after the sample is tilted from the vertical to horizontal position.

690 centistokes.

3.9 centistokes.

Fluid is clear and begins to flow immediately.

By comparison with the fluids of the examples, a commonly used hydraulic brake fluid composed of 40% by volume of the reaction product of castor oil and propylene glycol and 60% (methoxymethoxy) ethanol after being stored for 6 hrs. at

60 F. is turbid and begins to flow within 5 seconds after being tilted, but the fluid is much more viscous than that of Example 7.

The fluid of Example 6, when subjected to the lubrication or stroking test described in paragraph D12 of the Society of Automotive Engineers specification for hydraulic brake fluid, gives superior results, the metal parts used being free from excessive corrosion or galling.

The lubricating qualities of the hydraulic fluids of this invention may be further improved by the addition of assisting lubricants such as, defloccuiated graphite or the well known pressure lubricants such as, the phosphates of the aliphatic alcohols; tricresyl phosphate; ricinoleyl phosphate; t -e metal soaps such as sodium 01' p0 tassium oleate or palmitate and the like as well as other lubricating assistants.

Moreover, the more fluid compositions may be thickened by the addition of certain bodying agents which are compatible with the particular fluid such as, the metal stearates, and more especially aluminum, calcium, and magnesium stearates, cellulosic compounds such as regenerated and masticated cellulose, cellulose ethers and esters, and more particularly methyl and ethyl cellulose, glycol cellulose, gum tragacanth, cyclohexanol amine stearate, alginic acid salts and thelike. In using these thickening or bodying agents, they may be added in amounts ranging from to 4 parts per hundred parts by volume of the hydraulic fluid.

I claim:

1. A hydraulic fluid consisting of 40% by volume of di(ethylene glycol) glutarate and 60% by volume of isobutanol.

2. A hydraulic fluid having essentially this composition: a cli(glycol) ester of a dibasic aliphatic organic acid of the group consisting of glutaric acid and adipic acid and an organic solvent of the group consisting of low molecular weight alcohols, formals and ether alcohols therefor that boils above 200 F., there being present from 20 to 70% by volume of the ester based on the volume of the organic solvent.

3. A hydraulic fluid having essentially this composition: a di(glycol) ester of glutaric acid and an organic solvent of the group consisting of low molecular weight alcohols, formals and ether alcohols therefor that boils above 200 F., there being present from 20 to 70% by volume of the ester based on the volume of the organic solvent.

4. A hydraulic fluid having essentially this composition: a di(g1ycol) ester of adipic acid and an organic solvent of the group consisting of low molecular weight alcohols, formals and ether alcohols therefor boiling above 200 F., there be- 5. A hydraulic fluid having essentially this composition: a di(ethylene glycol) glutarate and an organic solvent of the group consisting of low molecular weight alcohols, formals and ether alcohols therefor that boils above about 200 F., there being present from 40 to by volume of the di(ethylene glycol) glutarate based on the volume of the organic solvent.

6. A hydraulic fluid having essentially this composition: a di(ethylene glycol) adipate and an organic solvent of the group consisting of low molecular weight alcohols, formals and ether alcohols therefor that boils above about 200 F., there being present from 40 to 60% by volume of the (methylene glycol) adipate based on the volume of the organic solvent.

7. A hydraulic fluid having essentially this composition: 40% by volume of di(ethylene glycol) adipate and 60% by volume of isobutanol.

8. A hydraulic fluid having essentially this composition: a di(glycol) ester of a dibasic aliphatic organic acid, of the group consisting of glutaric acid and adipic acid, and methoxymethoxy ethanol, there being present from 20 to by volume of the ester based on the volume of the methoxymethoxy ethanol.

9. A hydraulic fluid having essentially this composition: a di(glycol) ester of glutaric acid and methoxymethoxy ethanol, there being present from 20 to 70% by volume of the di(g1ycol) ester of glutaric acid based on the volume of the methoxymethoxy ethanol.

10. A hydraulic fluid having essentially this composition: a di(glycol) ester of adipic acid and KENNETH E. WALKER.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number Name Date 1,714,173 Kessler May 21, 1929 1,993,737 Graves et al. Mar. 12, 1935 2,293,309 Roblin, Jr. et a1. Aug. 18, 1942 I 2,402,754 Katz et al. June 25, 1946 2,435,619 Young et a1 Feb. 10, 1948 

1. A HYDRAULIC FLUID CONSSTING OF 40% BY VOLUME OF DI(ETHYLENE GLYCOL) GLUTARATE AND 60% BY VOLUME OF ISOBUTANOL. 